Self-amplified spontaneous emission for a single pass free-electron laser

SPARC (acronym of “Sorgente Pulsata ed Amplificata di Radiazione Coerente”, i.e. Pulsed and Amplified Source of Coherent Radiation) is a single pass free-electron laser designed to obtain high gain amplification at a radiation wavelength of 500 nm. Self-amplified spontaneous emission has been observed driving the amplifier with the high-brightness beam of the SPARC linac. We report measurements of energy, spectra, and exponential gain. Experimental results are compared with simulations from several numerical codes.

Figure 1

General SPARC layout. (A) High-brightness rf gun (BNL/UCLA/LCLS design). (B) Linac, composed by three traveling wave-type accelerating sections. (C) Transfer line. (D) Undulator, made by six permanent magnet modules, realized by ACCEL Gmbh. (E) Seeding equipment.

Figure 2

SPARC linac accelerator provides a final maximum beam energy of about 178 MeV. The first two linac accelerating structures are surrounded by two long solenoids (blue structures) providing the additional focusing (with a maximum field of 0.18 T) required to match the beam envelope to the linac, according to the invariant envelope conditions, when the linac operates in velocity bunching compression mode.

Figure 3

SPARC undulator consisting of six sections made by 75 periods of 2.8 cm each (77 periods including two termination periods), with gaps variable in the range from 25 to 8.4 mm (limited by the vacuum chamber) corresponding to a maximum undulator parameter $K_u\sim 2.2$ (see definition below) at the minimum gap.

Figure 4

Phase space of the e-beam measured with time/energy dispersion induced by rf-deflector/spectrometer dipole.

Figure 5

Beam current as a function of time along the microbunch. Error bars represent $\pm 1$ standard deviation evaluated by combining quadratically the relative local uncertainties on charge and bunch length ($\sim 5\%$).

Figure 6

Beam energy spread (A) and beam energy (B) as a function of time along the bunch. Error bars represent $\pm 1$ standard deviation on the local energy spread measure. The energy error bars are calculated as $\pm 3$ standard deviations of the energy measured at the specific longitudinal position. This parameter is related to the instrumental accuracy of the single shot measurement and does not reflect shot to shot energy fluctuations or drifts as indicated in Table .

Figure 7

Layout of the spectrometer and radiation diagnostics at the end of the undulator sequence.

Figure 8

Energy calibration including the contribution of the CCD detector, the reflectivity of the different gratings. A platinum coated grating (2400 grooves) covers the short wavelength range (black continuous) and two aluminum coated gratings, 1200 grooves (blue dotted) and 600 grooves (red dashed) cover the middle and long wavelength ranges, respectively.

Figure 9

Comparison of energy measured with pyroelectric detector (blue) and integrating the spectral images with the appropriate calibration factor (red). Error bars (red) correspond to 1 standard deviation over 100 shots. Numbers in parentheses indicate, for each data set, the undulator sections set at resonance.

Figure 10

Calculated evolution of the ${\beta }_x$ (continuous red) and ${\beta }_y$ (dotted blue) Twiss coefficients in the periodic FODO of length $L_p=256\mathrm{cm}$ composed by an undulator section of length $L_u=215.6\mathrm{cm}$ and a quadrupole ($Q$) in the drift between two adjacent modules.

Figure 11

Pulse energy vs longitudinal position in the undulator. Black symbols represent the measured data on the fundamental. Set (A) circles (average over 20 shots after the first undulator, $z\sim 5\mathrm{m}$), before optimization, set (B) squares (average over 50 shots), after optimizing laser to injector phase. Third harmonic (3h) triangles, were acquired with all the undulator gaps at resonance. Error bars represent $\pm 2$ standard deviations of the shot to shot fluctuations (error bar is missing after the first section where the spectrum is the result of two integrations over 100 s). The lines represent simulations with Perseo (red), Genesis 1.3 (brown), Medusa (blue), and Ginger (green). The third harmonic simulation data are represented by dash-dotted lines.

Figure 12

Histograms of radiation pulse energy, wavelength, linewidth, and spot size at the spectrometer slit for a set of 50 events corresponding to the highest energy measured, set (B) in Fig. 11.

Figure 13

Comparison between the experimental data on the fundamental harmonic (as in Fig. 11) and simulations done with Perseo in different conditions of matching and emittances: (A) emittances ($x/y$) ($2.5/2.9\mathrm{mm}\mathrm{\text{−}}\mathrm{mrad}$) and ${\sigma }_r=120\mu \mathrm{m}$; (B) emittances ($x/y$) ($2.5/2.9\mathrm{mm}\mathrm{\text{−}}\mathrm{mrad}$) and ${\sigma }_r=150\mu \mathrm{m}$; (C) emittances ($x/y$) ($2.0/2.0\mathrm{mm}\mathrm{\text{−}}\mathrm{mrad}$) and ${\sigma }_r=100\mu \mathrm{m}$; (D) emittances ($x/y$) ($2.0/2.0\mathrm{mm}\mathrm{\text{−}}\mathrm{mrad}$) and ${\sigma }_r=150\mu \mathrm{m}$.

Figure 14

Experimental linewidth of the FEL radiation (black circles) as a function of the longitudinal position in the undulator. The lines represent simulation data obtained with Perseo (red), Genesis 1.3 (brown), Medusa (blue), Ginger (green). The error bars represent $\pm 2$ standard deviation of the measured linewidth evaluated over 50 samples.

Figure 15

Evolution of the spectrum during the exponential gain growth. The number of sections closed is reported on the upper left corner. The vertical axis in each picture indicates the position on the vertical entrance slit of the spectrometer.